Why did God let this happen? I don't think we will ever know, and he didn't give the "green light" for it to happen, he just was bound by his own rule to not interfere with FREE WILL, that pesky thing. I think he cried when it happened, and he held our sould while it happened and after it happened and was trying to be the supportive and loving father. He didn't stop it, but failure to act does not mean he let it happen, he just respected his creation in man of free will, even when that will was evil and terrible.
I think in part we are who we are today because of what was done to us, and perhaps this happening to us was part of the "deal" we made with God when we were given our "mission." Maybe we wouldn't be able to fulfill our purpose without this happening to us, I know if this hadn't happened to us we wouldn't all be supporting one another and we probably wouldn't be as empathetic as we are.
Did God stop it? No, but did he just stand by and let it happen? No. He was there with us while we were abused, he was taking care of us and holding us in his arms as we endured the terrible acts of those sickos. He didn't stop it, but he didn't just stand by either, he held us and comforted us, at least he held and comforted our souls.
Peace be with you,
Impossible is just a big word thrown around by small men who find it easier to live in the world they've been given than to explore the power they have to change it. Impossible in not a declaration, it's a dare.